If your MIDI interface is displayed gray or translucent in the Audio MIDI Setup, then your computer is not able to communicate with it properly and will think that the device is offline. In most cases, unplugging and re-plugging the interface (while the computer is running) will fix this. If the problem persists, Audio MIDI Setup’s MIDI configuration might be damaged. In order to fix this, you will have to manually create a new MIDI configuration. Here’s how it works:
1. Open the Audio MIDI setup (AMS). To do so double-click your harddisk and go to Applications -> Utilities -> Audio MIDI Setup.
2. Click on the “Configuration” field. Most likely the field will display “default” if you have never created a custom configuration before.
3. Select “New Configuraton…”.
4. Enter a name for this new MIDI configuration, then click OK.
5. When a new MIDI configuration is created, it doesn’t contain any MIDI devices. Click on “Rescan MIDI” to get your MIDI devices back.
6. Your MIDI devices should now be displayed and should work as expected.